[llvm-dev] RFC: Allowing @llvm.objectsize to be more conservative with null.

[Sanjoy Das via llvm-dev]

Hi George,

On Mon, Jan 2, 2017 at 1:18 PM, George Burgess IV
<george.burgess.iv at gmail.com> wrote:
>> Continuing from above: in other words, since your use case is always
>> setting the `null-is-unknown` bit, can you re-define @llvm.objectsize
>> to have that semantic without adding a new parameter?
>>
>> That's not backward compatible, but there's a simple but conservative
>> update strategy.
>
> That would almost definitely be the simplest way forward, but I'm unsure how
> happy this would make other users of objectsize. In particular, our
> objectsize sanitizer will no longer catch any null-related issues like:
>
> struct Foo { int a; };
> int getA(struct Foo *f) { return f->a; }
> int getA2() { struct Foo *f = 0; return f->a; }

I guess that's another sign that the spec is wrong. :)

However, maybe we can use a trick here -- instead of lowering the
check to assert(ObjectSize(f) UGE 4) how about assert((1 +
ObjectSize(f)) UGT 4)?  The addition should always constant fold, and
if `f` is null then we'll assert((1 + (-1)) UGT 4) ==> assert(false).

-- Sanjoy

>
> // getA2 now traps rather than calling a special ubsan-check-failed function
>
> Grepping through, that's the only other direct user of objectsize that I can
> find. If we're okay with that loss in accuracy (and don't believe that we
> have other objectsize users that care), I'm happy to just alter objectsize's
> default behavior and see how it does.
>
>> However, none of this matters if we're not in a position to change the
>> specification.
>
> Sadly, we're not. :/
>
> On Mon, Jan 2, 2017 at 11:45 AM, Sanjoy Das <sanjoy at playingwithpointers.com>
> wrote:
>>
>> Hi George,
>>
>> On Mon, Jan 2, 2017 at 10:41 AM, George Burgess IV
>> <george.burgess.iv at gmail.com> wrote:
>> > Thanks for the comments!
>> >
>> >> Have you considered changing our existing behavior to match GCC's
>> >> builtin_object_size instead of adding a new parameter
>> >
>> > Yup! My issue with turning `i1 %min` into `i8 %flags` is that
>> > __builtin_object_size would get lowered like:
>> >
>> > __builtin_object_size(p, 0) -> @llvm.objectsize(i8* %p, i8 2)
>> > __builtin_object_size(p, 1) -> @llvm.objectsize(i8* %p, i8 2)
>> > __builtin_object_size(p, 2) -> @llvm.objectsize(i8* %p, i8 3)
>> > (__builtin_object_size(p, 3) doesn't actually get lowered)
>> >
>> > ...Which might be a bit surprising to some people. If we think that's a
>> > non-issue, I'm happy to take the simpler approach and use an i8.
>>
>> What I had in mind was:
>>
>>  __builtin_object_size(p, 0) -> @llvm.objectsize(i8* %p, i1 0)
>>  __builtin_object_size(p, 1) -> @llvm.objectsize(i8* %p, i1 0)
>>  __builtin_object_size(p, 2) -> @llvm.objectsize(i8* %p, i1 1)
>>  __builtin_object_size(p, 3) -> @llvm.objectsize(i8* %p, i1 1)
>>
>> and changing the specification of @llvm.objectsize to match.
>>
>> >> We'll have to have some <min>-awareness either in clang (to decide if
>> >> the <null-is-unknown> bit needs to be set) or in the middle end.  What
>> >> is your plan here?
>> >
>> > My plan was just to always set the `null-is-unknown` bit when lowering a
>> > call to __builtin_object_size in clang. If `min` is true, we treat
>> > unknown
>> > and null values identically in @llvm.objectsize, so whether
>> > `null-is-unknown` is set in that case shouldn't matter.
>>
>> Continuing from above: in other words, since your use case is always
>> setting the `null-is-unknown` bit, can you re-define @llvm.objectsize
>> to have that semantic without adding a new parameter?
>>
>> That's not backward compatible, but there's a simple but conservative
>> update strategy.
>>
>> >> However, since Malloc can return null:
>> >
>> > I think I was unclear: this behavior would only exist if
>> > @llvm.objectsize
>> > was actually handed null. I wasn't planning on changing how we'd handle
>> > memory allocation functions that may return null (GCC gives back 2 for
>> > __builtin_object_size(malloc(2), 0)). In other words, this
>> > `null-is-unknown`
>> > bit only makes the objectsize evaluator see `T* null` as though it was
>> > `call
>> > T* @opaque_user_defined_function()`, nothing else.
>>
>> I meant to say that it looks like the semantics of ObjectSizeMin(X) is
>> that "return the conservative minimum object size for all values that
>> X may take at runtime" (resp. ObjectSizeMax(X)).
>>
>> For instance this:
>>
>> void i(int c, volatile int* sink) {
>>   void* mem1 = malloc(20);
>>   void* mem2 = malloc(40);
>>
>>   *sink = __builtin_object_size(c ? mem1 : mem2, 0);
>>   *sink = __builtin_object_size(c ? mem1 : mem2, 2);
>> }
>>
>> is lowered to
>>
>>         movl    $40, (%rsi)
>>         movl    $20, (%rsi)
>>         ret
>>
>> by GCC.
>>
>> Applying the same logic to malloc(N), since it returns a location that
>> has N dereferenceable bytes or NULL, it follows that
>> ObjectSizeMin(malloc(N)) should return the smaller of
>> ObjectSizeMin(NULL) and ObjectSizeMin(MemoryLocOfNBytes) == the
>> smaller of ObjectSizeMin(NULL) and N.  Given that we want the result
>> of ObjectSizeMin(malloc(N)) == UMIN(ObjectSizeMin(NULL), N) to be N,
>> we'd want ObjectSizeMin(NULL) to be (unsigned)-1 for consistency.
>>
>> However, none of this matters if we're not in a position to change the
>> specification.
>>
>> -- Sanjoy
>>
>>
>> > This is also consistent with how @llvm.objectsize already acts: if the
>> > pointer it's given is the result of a call to `malloc(N)`, it'll return
>> > N
>> > regardless of the value of `min`.
>> >
>> > But yeah, I agree that some of the features of __builtin_object_size
>> > aren't
>> > entirely intuitive. :)
>> >
>> > On Mon, Jan 2, 2017 at 12:01 AM, Sanjoy Das
>> > <sanjoy at playingwithpointers.com>
>> > wrote:
>> >>
>> >> Hi George,
>> >>
>> >> Have you considered changing our existing behavior to match GCC's
>> >> builtin_object_size instead of adding a new parameter?  That may be
>> >> simpler overall.  There's also a clear upgrade strategy -- fold every
>> >> old style call to "<min> ? 0 : 1".
>> >>
>> >> You probably already know this, but GCC folds
>> >> builtin_object_size(0, 0) to -1 and builtin_object_size(0, 2) to 0.
>> >> We'll have to have some <min>-awareness either in clang (to decide if
>> >> the <null-is-unknown> bit needs to be set) or in the middle end.  What
>> >> is your plan here?
>> >>
>> >> I also found gcc's choice of folding builtin_object_size(0, 2) to 0 and
>> >> builtin_object_size(0, 0) to -1 somewhat odd; I'd have expected the
>> >> inverse.  This follows from the following "intuitive" rules
>> >>
>> >> ObjSizeMax(X) = UMAX(ObjSizeMax(A), ObjSizeMax(B))
>> >> ObjSizeMin(X) = UMIN(ObjSizeMin(A), ObjSizeMin(B))
>> >>
>> >> (where X is a value that can either be A or B at runtime)
>> >>
>> >> and that we want to fold
>> >>
>> >> ObjSizeMax(Malloc(N)) = ObjSizeMin(Malloc(N)) = N
>> >>
>> >> However, since Malloc can return null:
>> >>
>> >> ObjSizeMax(Malloc(N)) = UMAX(N, ObjSizeMax(NULL)) = N
>> >> ObjSizeMin(Malloc(N)) = UMIN(N, ObjSizeMin(NULL)) = N
>> >>
>> >> and for that to be true ObjSizeMax(NULL) =
>> >> builtin_object_size(NULL, 0) needs to be 0 and ObjSizeMin(NULL) =
>> >> builtin_object_size(NULL, 2) needs to be (unsigned)-1.
>> >>
>> >> However, I'm not sure if it is up to us to change that; given the very
>> >> motivation of this thread is GCC compatibility.
>> >>
>> >> -- Sanjoy
>> >>
>> >> On Sun, Jan 1, 2017 at 10:03 PM, George Burgess IV via llvm-dev
>> >> <llvm-dev at lists.llvm.org> wrote:
>> >> > Happy New Year ping. :)
>> >> >
>> >> > Will ping again on Wednesday. If I don't get comments by EOD
>> >> > Thursday,
>> >> > I'll
>> >> > assume everyone's OK with this and put together a patch.
>> >> >
>> >> > On Wed, Dec 21, 2016 at 11:44 AM, George Burgess IV
>> >> > <george.burgess.iv at gmail.com> wrote:
>> >> >>
>> >> >> tl;dr: We'd like to add a bit to @llvm.objectsize to make it
>> >> >> optionally
>> >> >> be
>> >> >> conservative when it's handed a null pointer.
>> >> >>
>> >> >> Happy Wednesday!
>> >> >>
>> >> >> We're trying to fix PR23277, which is a bug about how clang+LLVM
>> >> >> treat
>> >> >> __builtin_object_size when it's given a null pointer. For
>> >> >> compatibility
>> >> >> with
>> >> >> GCC, clang would like to be able to hand back a conservative result
>> >> >> (e.g.
>> >> >> (size_t)-1), but the LLVM intrinsic that clang lowers
>> >> >> __builtin_object_size(p, N) to, @llvm.objectsize, only hands back 0
>> >> >> when
>> >> >> given a null pointer. Because it's often assumed that
>> >> >> __builtin_object_size
>> >> >> always folds to a constant, handling this only in clang may be
>> >> >> tricky:
>> >> >> if we
>> >> >> emit the IR equivalent to ((p) ? __builtin_object_size(p, N) :
>> >> >> -1ULL)
>> >> >> and
>> >> >> LLVM can't fold away the null check, we've failed to emit a
>> >> >> constant.
>> >> >>
>> >> >> So, the best path forward that I can see is to add a "null is
>> >> >> unknown
>> >> >> size" bit to @llvm.objectsize, much like the "min" bit it already
>> >> >> has.
>> >> >> If
>> >> >> this bit is true, null would be treated like an opaque pointer.
>> >> >> Otherwise,
>> >> >> @llvm.objectsize would act no differently than it does today.
>> >> >>
>> >> >> If we like this approach, I'm assuming it would be best to have this
>> >> >> bit
>> >> >> as a third argument to @llvm.objectsize, rather than making the
>> >> >> second
>> >> >> argument an i8 and using it as a bag of bits.
>> >> >>
>> >> >> All thoughts/comments/alternative approaches/etc. highly
>> >> >> appreciated.
>> >> >> :)
>> >> >>
>> >> >> Thanks (and Happy Holidays)!
>> >> >> George
>> >> >
>> >> >
>> >> >
>> >> > _______________________________________________
>> >> > LLVM Developers mailing list
>> >> > llvm-dev at lists.llvm.org
>> >> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>> >> >
>> >>
>> >>
>> >>
>> >> --
>> >> Sanjoy Das
>> >> http://playingwithpointers.com
>> >
>> >
>
>